1. Field of the Invention
The present invention relates to a liquid film forming method for forming a film by spirally dropping liquid on a substrate, and a method of forming a solid film using the same.
2. Description of the Related Art
In the spin coating method hitherto employed in the lithographic process, most of the liquid dropped on a substrate is discharged outside of the substrate. Only several percent of the liquid supplied on the substrate is actually used for forming a film, and the most liquid is wasted. Besides, much chemical solution is discharged, and it has adverse effects on the environments. Further, in a square substrate or a circular substrate of large diameter of 12 inches or more, an air turbulence occurs on the periphery of the substrate, and the film thickness is not uniform on the peripheral portion.
A technology for applying uniformly on the entire surface of the substrate without wasting chemical liquid is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-220428. This publication teaches a method of dropping a resist liquid from multiple nozzles arranged in a row, and blowing gas or liquid to the film forming surface from behind, thereby obtaining a uniform film.
Jpn. Pat. Appln. KOKAI Publication No. 6-151295 discloses a technology intended to obtain a uniform film by dropping a resist liquid on a substrate. This publication teaches a method of spraying a chemical solution to the substrate from multiple injection ports provided in a bar.
Jpn. Pat. Appln. KOKAI Publication No. 7-321001 discloses a technology of coating while moving relatively with the substrate, by using a spray head having multiple jet holes for spraying a resist liquid.
In these application apparatuses, it is commonly intended to form a uniform film by arranging plural dropping or spraying nozzles in a row, and scanning them along the substrate surface.
Aside from these coating methods of using the apparatus having plural nozzles, a method of forming a liquid film by using one liquid discharge nozzle and scanning on the substrate is also proposed. In this method, however, the processing time per substrate may be very long depending on the nozzle operating technique, or the consumption of chemical solution may be notably increased.
In order to solve these problems, a technique of applying by spirally supplying the chemical solution is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-77326. In this method, “it is preferred to apply, as the coating condition, by moving the nozzle unit in the wafer diametral direction (for example, X-direction) while rotating the wafer at low frequency (for example, 20 to 30 rpm). It is also important to “keep constant the relative speed of the wafer and nozzle unit.” That is, it is defined that the linear speed of the nozzle should be constant.
When moving the nozzle unit at a constant speed, to make the linear speed constant, the rotational frequency at the inner side should be higher than at the nozzle periphery. For example, in the case of a wafer of diameter of 200 mm, even if the rotational frequency is 30 rpm at radius of 100 mm, the rotational frequency is proportional to the reciprocal number of the diameter, and in the portion of radius of 1 mm or less, the rotational frequency must be 3000 rpm or more. While the wafer is rotating at 3000 rpm, even if liquid application is started from the center of the substrate, the chemical solution is instantly discharged outside of the substrate.
When the wafer is rotated at a constant low frequency, the nozzle moving speed in the center of the substrate is extremely fast. Even if the liquid is moved by applying vibration after application, the nozzle is not moved sufficiently, and the chemical solution is not applied in a central area. As a result, a uniform film cannot be formed. Thus, when the chemical solution is discharged while keeping the linear speed constant, a liquid film of uniform thickness cannot be formed.